Pressure relief devices - oil control

Author: Scott Glasson


Power transformers are typically insulated with a transformer oil such as mineral oil, natural ester oil or silicon. The oil serves multiple purposes. It provides cooling via natural convection, as well as additional electrical insulation. Due to the nature of transformers, the oil is going to heat up.  Going back to our Thermodynamics classes, we know the ideal gas law is PV=nRT (Pressure x Volume = moles of substance x gas constant x temperature). As the temperature increases, so does the volume and the pressure. Transformer tanks are not built to be considered pressure vessels, so they need to be able to ease the built up pressure with a pressure relief device (PRD).


The pressure relief device is a spring loaded mechanism that vents excessive pressure from your transformer. Chances are that if your equipment utilizes a conservator for the oil preservation, or your transformer’s oil level is too high, your PRD has seeped oil. It may only release a gallon of oil, but that one gallon can cover quite a bit of surface area. To add to your frustration, you probably will have oil on your radiators, fans, control cabinet, the top of the transformer, the ground, and worst case scenario, yourself.


A good PRD can literally and figuratively circumvent these issues and provide you with several options with safety and the environment in mind. Shrouds can be included to cover the pressure relief device for directional ventilation. Additional piping can be included to route the exit point. These are both great add-ons. Considering that the PRD is mostly comprised of a spring and gasket material, the shroud will also protect the components from the elements and prolong it’s life. The last thing you want to encounter on top of a leaking transformer is a nest of wasps or spiders. Be sure to get a bug screen to make your job even safer. All in all, it is a small investment for the return.


The long story short is your have relatively cheap options for this additional safety device. If you are having a new transformer built, you can quickly add it into your specification, adding a shroud over your PRDs. If you have existing transformers, you still have options. Depending on your device, your transformer can likely be retrofitted with a shroud or replaced with a new PRD that includes a shroud.